As it is known, a compressor is a machine that is capable of raising the pressure of a compressible fluid (gas) through the use of mechanical energy. Amongst the various types of compressors used in process plants in the industrial field, there are so called centrifugal compressors, in which energy is provided to the gas in the form of centrifugal acceleration due to rotation, generally controlled by a driver (electric motor or steam turbine), of a member called centrifugal impeller or wheel.
Centrifugal compressors can be provided with a single impeller, in the so called single-stage configuration, or with many impellers arranged in series, in this case called multistage compressors. More precisely, each stage of a centrifugal compressor is normally made up of a suction conduit for the gas to be compressed, of an impeller, which is capable of providing kinetic energy to the gas, and of a diffuser, the task of which is that of converting the kinetic energy of the gas in outlet from the impeller into pressure energy.
In alternative types of compressors, on the other hand, fluid is compressed by one or more pistons which can move with alternate motion inside a respective cylinder. The fluid to be compressed is sucked into the cylinder through one or more suction conduits, whereas the compressed fluid is delivered from the cylinder towards one or more delivery conduits. Commonly, the piston or the pistons of an alternate compressor are actuated by electric motors or by internal combustion engines, through a crank shaft for transmitting the motion and a conventional rod-crank mechanism.
In gas compression plants, be they provided with one or more centrifugal or axial type compressors, so called antisurge systems are often present, suitably designed and sized to allow the compressor to operate even if there is a very low gas flow rate. Indeed, as it is known, the flow rate of gas entering into a compressor must not drop below a certain limit value, otherwise, the compressor finds itself operating in surge conditions.
Surge is a phenomenon of instability in the gas flow that occurs when the compressor reaches the point in which the pressure-flow rate curve becomes flat. From such an operating condition, if the flow rate decreases further, the compressor is not capable of producing enough pressure to oppose the downstream resistance. More precisely, the delivery pressure of the compressor is lower than the pressure in the plant downstream of the compressor itself. This condition can cause the gas flow to be reversed through the compressor. This condition is particularly dangerous for the mechanical integrity of the machine, due to the vibrations and to the forces induced by the reversing of the flow rate. For this reason the antisurge system allows the recirculation of an amount of gas which is sufficiently high, so as to avoid that the machine operates in surge condition.
Antisurge systems can be provided with both closed loop recirculation pipes, with relative antisurge valves, as well as with relief valves placed in the portion of the plant downstream of the compressor. In compression plants provided with closed loop recirculation pipes, when the gas flow rate drops below the critical value which causes the surging phenomenon to occur, suitable valves send the gas back, through the recirculation pipes, from the delivery conduit to the suction conduit. The work provided by the compressor to the gas is thus dissipated inside the recirculation pipes, mainly through the antisurge valve, and the heat produced is normally eliminated by means of a cooling device. In other words, when a compressor is operating in recirculation conditions, the head (i.e. pressure increase) that such a compressor is capable of providing to the gas must be dissipated in the recirculation pipes. Normally, this occurs through the opening of the antisurge valve to reduce the gas pressure or, to use the jargon, to “laminate” the gas.
If the antisurge system recirculates the entire flow rate of the gas that passes through the compressor, the overall mechanical energy provided to the compressor by the relative actuation motor is converted into heat, the removal of which is helped by the cooling device present in the plant. The energy necessary for the compressor to operate in surge condition is thus lost, according to the percentage of recycled flow rate, since the heat given up by the gas in the recirculation pipes is normally difficult to recover.
Indeed, to this day, the only way to partially reduce the energy losses when the compressor operates in recirculation conditions is that of minimizing the quantity of recycled gas by acting upon the antisurge algorithm, i.e. by modifying the size of the recirculation pipes and of the relative antisurge valve, or by modifying the operating characteristic of the compressor by means of adjustment systems, like systems for varying the speed, mobile blades, etc. In any case, these systems, when they can be applied to plants for conveying gas or hydrocarbons, do not allow the operative minimum flow rate to be reduced at will.
The general purpose of the present invention is therefore that of making an energy recovery system in a gas compression plant that is capable of overcoming the aforementioned problems of the prior art.
In particular, a purpose of the present invention is that of making an energy recovery system in a gas compression plant capable of transforming, at least partially, the losses of energy that occur when the compressor or the compressors of the plant operate in gas recirculation conditions into useful work.
Another purpose of the invention is that of making an energy recovery system in a gas compression plant capable of improving the overall performance of the plant itself, at the same time keeping the consumption of fuel and heat emission low, all to the benefit of reduction of the greenhouse effect.
These and other purposes according to the present invention are achieved by making an energy recovery system in a gas compression plant as outlined in claim 1.
Further characteristics of the invention are highlighted in the dependent claims, which are an integral part of the present description.
It should be noted that in each figure, same numbers correspond to the same systems or components of the previous and/or following figures.